It has long been known that characteristics of the Earth’s orbit (its eccentricity, the degree to which it is tilted, and its “wobble”) are slightly altered on timescales of tens to hundreds of thousands of years. Such variations, collectively known as Milankovitch cycles, conspire to pace the timing of glacial-to-interglacial variations.

Despite the immense explanatory power that this hypothesis has provided, some big questions still remain. For one, the relative roles of eccentricity, obliquity, and precession in controlling glacial onsets/terminations are still debated. While the local, seasonal climate forcing by the Milankovitch cycles is large (of the order 30 W/m2), the net forcing provided by Milankovitch is close to zero in the global mean, requiring other radiative terms (like albedo or greenhouse gas anomalies) to force global-mean temperature change.

The last deglaciation occurred as a long process between peak glacial conditions (from ~26-20,000 years ago) to the Holocene (~10,000 years ago). Explaining this evolution is not trivial. Variations in the orbit cause opposite changes in the intensity of solar radiation during the summer between the Northern and Southern hemisphere, yet ice age terminations seem synchronous between hemispheres. This could be explained by the role of the greenhouse gas CO2, which varies in abundance in the atmosphere in sync with the glacial cycles and thus acts as a “globaliser” of glacial cycles, as it is well-mixed throughout the atmosphere. However, if CO2 plays this role it is surprising that climatic proxies indicate that Antarctica seems to have warmed prior to the Northern Hemisphere, yet glacial cycles follow in phase with Northern insolation (“INcoming SOLar radiATION”) patterns, raising questions as to what communication mechanism links the hemispheres.

There have been multiple hypotheses to explain this apparent paradox. One is that the length of the austral summer co-varies with boreal summer intensity, such that local insolation forcings could result in synchronous deglaciations in each hemisphere (Huybers and Denton, 2008). A related idea is that austral spring insolation co-varies with summer duration, and could have forced sea ice retreat in the Southern Ocean and greenhouse gas feedbacks (e.g., Stott et al., 2007).

Based on transient climate model simulations of glacial-interglacial transitions (rather than “snapshots” of different modeled climate states), Ganopolski and Roche (2009) proposed that in addition to CO2, changes in ocean heat transport provide a critical link between northern and southern hemispheres, able to explain the apparent lag of CO2 behind Antarctic temperature. Recently, an elaborate data analysis published in Nature by Shakun et al., 2012 (pdf) has provided strong support for these model predictions. Shakun et al. attempt to interrogate the spatial and temporal patterns associated with the last deglaciation; in doing so, they analyze global-scale patterns (not just records from Antarctica). This is a formidable task, given the need to synchronize many marine, terrestrial, and ice core records.More »

As many will have already heard, our colleague, RC co-founder and friend Michael Mann will receive the Oeschger medal from the European Geosciences Union this week in Vienna. We are delighted to announce this and to congratulate Mike.

Hans Oeschger was a Swiss scientist originally trained as a nuclear physicist. His name is well known in climate science, especially because of his discovery, with Willi Dansgaard, of the Dansgaard-Oeschger events (the rapid climate changes during the last glacial period, first observed in Greenland ice cores). He was even better known in the radiocarbon research community as famously having developed one of the first instruments (the “Oeschger counter”) for measuring carbon-14. This paved the way for determining the age of very small organic materials, including samples from deep-sea sediment cores, which eventually led to the validation of the Milankovitch theory of ice ages. Oeschger and his colleagues in Bern were the first to measure the glacial-interglacial change of atmospheric CO2 in ice cores, showing that atmospheric concentrations of CO2 during the glacial period was 50% lower than the pre-industrial concentration, a result predicted by Arrhenius nearly a century earlier. Oeschger may thus be credited with work that was critical to validating two of the most important theories in science: the role of CO2 in climate change, and the role of changes in the earth’s orbit. Oeschger was also an accomplished musician, and was known to join colleagues in playing chamber music at the International Conference on Radiocarbon.

Oeschger left rather large shoes to fill, and it is a great honor for Mike Mann to win an award bearing Oeschger’s name. Most everyone will probably assume that the award is for Mike’s well known “hockey stick” work. No doubt this is part of it, but the Oeschger award has never been given simply for the publication of one study, but rather for a career’s-worth of outstanding achievements. Most of the previous medalists are a good deal more senior than Mike Mann, and include paleoceanographer Laurent Labeyrie, limnologist Francoise Gasse, ice core pioneers Dominique Raynaud and Sigfus Johnsen and number of other major names in the climate and paleoclimate research, including RC’s own Ray Bradley.

Mike’s work, like that of previous award winners, is diverse, and includes pioneering and highly cited work in time series analysis (an elegant use of Thomson’s multitaper spectral analysis approach to detect spatiotemporal oscillations in the climate record and methods for smoothing temporal data), decadal climate variability (the term “Atlantic Multidecadal Oscillation” or “AMO” was coined by Mike in an interview with Science’s Richard Kerr about a paper he had published with Tom Delworth of GFDL showing evidence in both climate model simulations and observational data for a 50-70 year oscillation in the climate system; significantly Mike also published work with Kerry Emanuel in 2006 showing that the AMO concept has been overstated as regards its role in 20th century tropical Atlantic SST changes, a finding recently reaffirmed by a study published in Nature), in showing how changes in radiative forcing from volcanoes can affect ENSO, in examining the role of solar variations in explaining the pattern of the Medieval Climate Anomaly and Little Ice Age, the relationship between the climate changes of past centuries and phenomena such as Atlantic tropical cyclones and global sea level, and even a bit of work in atmospheric chemistry (an analysis of beryllium-7 measurements). Mike’s earliest work, as a physicist, involved studying the behavior of liquids and solids, and trying to understand phenomena such as the structural ordering of high temperature superconductors. In the earth sciences, he has published on topics as varied as the recovery from the KT-boundary mass extinction event and the factors driving long-term changes in the volume of the Great Salt Lake. He has studied and published on the impacts of historical and projected climate change on everything from the behavior of the Asian Summer Monsoon, to Atlantic Hurricanes, to rainfall patterns in the U.S. And for those interested in the hard-nosed statistics by which a scientist’s productivity gets measured, a quick check on the ISI web site will tell you that he has an “H Index” of 40 (that means that 40 of his papers have been cited at least 40 times), more than twenty of his papers have over 100 citations each, and two have over 700. Those are high numbers by any comparison.

But back to the hockey stick. Mike has weathered some rather intense scrutiny and criticism over the years, mostly over the details of a paper nearly 15 years old. Yet the basic conclusions of the “hockey stick” remain, and indeed have been strengthened by subsequent work. Most will be aware, for example, that the conclusion that the past few decades are likely the warmest of the past millennium — i.e. the conclusion of the best-known of Mike’s papers in Nature and Geophysical Research Letters –has never been seriously challenged. But well beyond the simple fact of having been right, Mike’s work was seminal, like Oeschger’s, in playing a pivotal role in launching an entirely new field of study. Although some earlier work along similar lines had been done by other paleoclimate researchers (Ed Cook, Phil Jones, Keith Briffa, Ray Bradley, Malcolm Hughes, and Henry Diaz being just a few examples), before Mike, no one had seriously attempted to use all the available paleoclimate data together, to try to reconstruct the global patterns of climate back in time before the start of direct instrumental observations of climate, or to estimate the underlying statistical uncertainties in reconstructing past temperature changes. Since Mike’s pioneering work (starting in 1995), hundreds of papers have adopted the basic approach he pioneered, and numerous PHD projects have been launched to try to improve upon it. Methods have improved of course, and no doubt will improve further (paleoclimate reconstruction using weather forecast data assimilation methods is the latest and most promising recent development). That Mike is a co-author on many of the latest and most innovative publications in this area — with dozens of different people — attests to the groundbreaking nature of his work.

We look forward to seeing Mike’s award lecture in Vienna, and we offer our heartfelt congratulations to a well-deserved honor. And while we are at it, we should congratulate Mike in advance for his election as a Fellow of the American Geophysical Union; that honor will be bestowed this fall in San Francisco.

P.S. For those at EGU, you should also check out glaciologist Ian Joughin’s award lecture (Wednesday evening) for the Agassiz medal, for his important work in documenting and understanding the acceleration of Antarctica and Greenland’s glaciers.

Just a quick note to point out that the HadCRUT4 data are now fully available for download. Feel free to discuss (or point to) any analyses you’d like to see done in the comments, and perhaps we’ll update this post with the more interesting ones.

We have just passed the annual maximum in Arctic sea ice extent which always occurs sometime in March. Within a month we will reach the annual maximum in Arctic sea ice volume. After that, the sea ice will begin its course towards its annual minimum of both extent and volume in mid-September. This marks the beginning of the ritual of the annual sea ice watch that includes predictions of the extent and rank of this year’s sea ice minimum, as well as discussion about the timing of its eventual demise. One of the inputs into that discussion is the “PIOMAS” ice-ocean model output of ice volume – and in particular, some high-profile extrapolations. This is worth looking at in some detail.

Sometimes it helps to take a step back from the everyday pressures of research (falling ill helps). It was in this way we stumbled across Hansen et al (1981) (pdf). In 1981 the first author of this post was in his first year at university and the other just entered the KNMI after finishing his masters. Global warming was not yet an issue at the KNMI where the focus was much more on climate variability, which explains why the article of Hansen et al. was unnoticed at that time by the second author. It turns out to be a very interesting read.More »